Generally, a vehicle's automatic transmission (AT) automatically shifts into a target shift speed based on vehicle-speed, throttle opening, etc Such ATs include on-coming elements and off-going elements. Accordingly, while the shift speed is shifted into the target shift speed, the off-going elements are disengaged by a hydraulic control, and the on-coming elements are engaged by the hydraulic control.
In particular, these ATs include a torque converter having a damper clutch, and a power-train having a shift gear mechanism, as the on-coming or the off-going elements. However, vehicles with ATs have a problem in that fuel is unnecessarily wasted. That is, since energy is consumed by a slip in the torque converter, the vehicles with ATs have a problem in that fuel mileage is decreased as compared to vehicles with manual transmissions.
In addition, because of such a decrease of fuel mileage, the vehicles with ATs expel exhaust gas that includes noxious substances. This exhaust gas leads to environmental pollution.
Accordingly, to decrease such fuel waste, damper clutch control methods of vehicle automatic transmissions have been provided. Such damper clutch control methods of vehicle automatic transmissions engage a damper clutch during an inertia travel under a power-off state (when the driver's foot is removed from the gas), in order to intercept the slip in the torque converter, that is, in order for an engine RPM to maintain equally to a turbine RPM.
According to the method, since an engine output-shift and an AT input-shift are engaged with each other by the damper clutch, slip in the toque converter is not generated. Since the slip of the torque converter is not generated, fuel mileage is improved. Furthermore, since, if the damper clutch is engaged during the inertia travel under a power-off state, torque of the AT is transmitted into an engine, and the engine RPM is slowly decelerated. Accordingly, since a fuel cut time becomes longer, the fuel mileage can be enhanced.
The damper clutch control method of an automatic transmission for vehicles will hereinafter be explained in detail with reference to FIG. 4. Firstly, while a vehicle is traveling at a second speed in a power-on state, an engine RPM A is shown higher than a turbine RPM B. After this, the power-off state is initiated and the shift speed is up-shifted into a third speed.
After this, when various conditions of an oil temperature, an engine RPM, and a turbine RPM, etc., satisfy an engagement condition of the damper clutch, the damper clutch is engaged. After a predetermined interval, the shift speed is up-shifted into a fourth speed. However, while the shift speed is being up-shifted into the fourth speed, the engaged damper clutch is disengaged in order to enhance shift feel.
However, the damper clutch control method of an automatic transmission has the following problems. Firstly, since the engaged damper clutch is disengaged while the shift speed is being up-shifted into the fourth speed, the engine RPM is rapidly decelerated in comparison with the turbine RPM. Accordingly, as shown in FIG. 4, a difference between the engine RPM and the turbine RPM becomes larger. Consequently, a problem occurs that the damper clutch is not engaged while the vehicle is traveling at the fourth speed.
In addition, since an exchange of information does not occurr between an engine control unit (ECU) and an AT control unit (TCU), a fuel-cut control and the damper clutch control are independently executed. Accordingly, a problem occurs that efficiency of this method is decreased.
In addition, there is a range that the damper clutch control can not be executed. For example, the prior damper clutch control method is not applied while the vehicle is traveling in the power-on state, and while the shift speed is being up-shifted into any speed in the power-off state. Accordingly, since the application range of the prior damper clutch control method is very limited, a problem occurs that efficiency of the prior method is decreased.
In addition, since the damper clutch control is executed during the power-off state, oil generated by an oil pump is very small. Accordingly, a problem occurs in that time for controlling the damper clutch is retarded.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.